Hitherto, there are many instances where contour extraction processing for extracting a contour shape of an object included in plural picture image data constituting a moving picture image are used in the field of picture image processing, e.g., CAD, CG technologies, etc. Among these picture image processing methods, when picture image synthesis processing is carried out, there is the processing that generates key signals from the contour shape of the object, and it is important to generate precise or accurate key signals, i.e., precise contour shape. In this case, the key signal is information for cutting off the area of the foreground object to be synthesized and is also called a mask.
The contour extraction processing is required to have interactive operability as in the case where an operator gives in detail information such as contour position or direction, etc. with respect to the object. As an example of such contour extraction processing, there is processing to designate plural points on the contour of the object to prepare shapes between respective points, processing to operate control points of a parametric curve indicating a counter shape, and processing to directly input the contour shape by mouse, etc. As examples of these contour extraction processing methods, there are the known publications “Intelligent Scissors for Image Composition” (Eric N. Mortensen and William A. Barrett, Computer Graphics Proceedings, Annual Conference Series, 1995, ACM SIGGRAPH, pp. 191–198), “Image contour detection method” (Japanese Patent Application Laid Open No. 152481/1992 publication), and “Cutting mask preparation method and cutting mask preparation apparatus” (Japanese Patent Application Laid Open No. 254854/1992 publication).
Moreover, in the contour extraction processing, it is required to extract a precise contour shape with respect to all picture images constituting a moving picture. For example, in cinemas or television picture images, several hundreds of key signals are required for generating images of several seconds, and the quantity of processing is vast. For this reason, the contour extraction processing used in the field of cinema, etc. is desired (required) that a more precise contour shape can be prepared by a lesser number of operations. This results from the fact that, in the conventional contour extraction processing, processing for preparing contour shapes with respect to respective frames constituting a moving picture is processing that requires extremely much time and labor.
In view of the above, as a method of preparing a contour shape by a more simple procedure, there was proposed a method in which, on the premise that, in moving picture images having continuous property, a contour shape at the start frame changes with passage of time so that it changes to a contour shape at the end frame, an intermediate shape of a contour at the intermediate frame is prepared by interpolation processing from the contour shape at the start frame and the contour shape at the end frame.
In accordance with such an interpolation processing, as shown in FIG. 1, plural points constituting contour shape are caused to have a one-to-one correspondence relationship at the contour shape of the start frame and the contour shape of the end frame. Namely, when it is assumed that the curve indicating contour shape at the start frame (time t=0) is curve C0, the curve indicating contour shape at the end frame (time t=1) is curve C1, and the curve changes in point of time, it can be considered that curve C0 at time t=0 changes to curve C1 at time t=1. In view of the above, when time t at which interpolation of the two curves is carried out is assumed to be T, and the coordinate at the point on the curve C0 is put by A and coordinate at the point on the curve C1 corresponding to the coordinate A is put by B, the coordinate C at curve Cx which takes required intermediate form can be represented as follows.C=T·A+(1−T)·B In the conventional interpolation processing, contour curve Cx at the intermediate frame (t=T) was prepared from a train of points determined by such interpolating calculation.
Further, it is assumed that contour shapes with respect to respective frames are constituted by a plural third-order Bezier curve. The third-order Bezier curve is defined byQ(t)=((1−t)3)M+3t((1−t)2)N+3(t2)(1−t)O+(t3)P (0≦t≦1)as indicated by the article “Computer Graphics PRINCIPLE AND PRACTICE SECOND EDITION in C” (Foley, van Dam, Feiner, Hughes, ADDISON WESLEY, 1996, ISBN 0-201-84840-6). In this formula, M, N, O, P are coordinates indicating points on the two-dimensional plane wherein M and P are end points of a line segment, and N, O are control points. An example of the line segment of the third order Bezier curve represented by the above-mentioned formula is shown in FIG. 2. In FIG. 2, the curve indicated by the solid line is the third-order Bezier curve, wherein M, P indicated by points of the black circle are end points, and points N, O indicated by the white circle are control points.
In accordance with the formula which defines the above-described Bezier curve, the locus of the third-order Bezier curve, i.e., shape is represented by coordinates of end points and control points indicated by points M, N, O, P and change of time t in FIG. 2.
When the constituting unit of the third-order Bezier curve shown in FIG. 2 is assumed to be a segment, it can be said that a single curve indicating contour shape is ordinarily composed of plural segments as shown in FIG. 3. In this case, end points of adjacent segments are shared so that one curve is constituted as the entirety.
In the above-described conventional contour extraction processing, in the case where the intermediate shape is determined from contour shapes of frames before and after a point in time, interpolation was carried out in the state where control points R and end points S constituting segments in FIG. 3 are caused to have a one-to-one correspondence relationship.
However, at frames before and after the point in time, there are instances where the contour shape does not linearly move.
In the above-mentioned method, in the case where a contour shape moves non-linearly, the shape is different from when the contour shape was prepared.
In order to avoid such problems, it is conceivable to carry out work to divide an interval of points in time between the time of start and the time of end into narrower ranges to carry out interpolation processing of the shape for a second time, or to manually modify the shape, etc. For this reason, there took place the problem that labor of the user was required in preparing the shape, and there took place fine unevenness of the shape because the shape that is prepared discontinuously changes in the time axis direction owing to manual modification.